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Monday, June 9, 2014

BUFFER SYSTEMS AND THEIR ROLES IN REGULATING THE pH OF BODY FLUIDS

A buffer is a mixture of a weak
acid and salt of its conjugate base that resists the change in pH upon the
addition of acid or base. If acid and base components of buffer are equal, the
pH is equal of pK. Generally buffer
works best at pH ±1 unit of
its pK. Buffer work best when the
ratio of acid : base is within the range of 10 : 1 to 1 : 10. Buffers are also
effective at higher concentration.Bicarbonate/carbonic
acid buffer system:

This
is the most important and predominant buffer system in plasma. It has pK of
6.1. The ratio of base to acid is 20:1. The effectiveness of this system is due
to its high concentration (>20 mmol/L) and lung can easily dispose off and
retain CO2. In addition renal tubules can increase or decrease the
rate of reclamation of bicarbonate from the glomerular filtrate. Other non
bicarbonate buffer system are present at <10 mmol/L concentration. The
buffer value β is the amount of base required to cause a
change in pH of one unit. The buffer value of this system is 55.6 mmol/L. Phosphate buffer
system:At
physiological pH the ratio of cHPO42-/cH2PO4-
is 4/1 (pK of 6.8). This is also found in plasma and erythrocyte but accounts
only 5% of non bicarbonate buffer system in plasma. 2,3-DPG in
RBC at about 4.5 mmol/l accounts for 16% non bicarbonate buffer system in
erythrocyte.

Plasma protein buffer system:

Protein
especially albumin accounts for greater proportion (95%) of non bicarbonate
buffer in plasma. The most important buffer groups of proteins are imidazole
groups of histidine (pK about 7.3) and each albumin contains 16 histidines.

The reaction to the right is the main mechanism of non
bicarbonate buffer system and the component Pr‑/HPr represents all
non bicarbonate buffer system. The main purpose of non bicarbonate buffer
system is to maintain constant H+.

Hemoglobin
buffer system:

The buffer value of non bicarbonate
buffers of erythrocyte fluid is about 63 mmol/L. Hemoglobin accounts for major
part (53 mmol/L), with remainder being mainly caused by 2,3-DPG. The imidazole group
of hemoglobin is the most important buffering groups. When oxygenated, H+
ions are liberated from Hb a phenomenon called the Haldane effect. Similarly
release of oxygen by binding to hydrogen is called Bohr Effect.